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Guest Kevin Farley

An easy one for y'all I'm sure

7 posts in this topic

OK, this is probably a dumb ? but I'll ask it anyway. But first, let me say that I'm self taught in AB's so please go easy on me. Ok, I work in a building with around 50 SLC 50X PLCs in it. I've no problems connecting to the tools, going online, or even programming them when needed. Now, I'm VERY familure with the hardware on these machines so it's not a problem normally to open the logix, find the general area that the machine is having a problem in, and checking to see if a bit is on or off. Here's my deal though, I know there is a way that you can tell what the last rung that was executed was, but I don't know how. This would make it a lot easier for me, as I normally have to guess "OK, this has happened on the machine, so it must be this step..." As you can guess, this only leads to about an 80% success rate. So, I guess what I'm getting at is: How do you find out what rung the program is waiting on? Kevin

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Oops, sorry... Forgot to mention that I'm using RSLogix 6 pro...

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The processor does not wait for a rung to be true before going to the next one. It continuously scans all rungs sequentially and updates bits, inputs, outputs, etc as it goes along. If you are using a step register perhaps moving an integer to a register after a step is completed might help to see where the machine stopped. If it is a processor fault that is stopping the machine trying checking the status (S) file under the Proc tab it should show the fault\powerdown rung. But this is not always help for minor error bits.

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OK, that makes sense. I guess what I ment to say was; Is there a way to find out what rung last evaluated to true. Someone had once suggested looking for a 1 in the datatables, but upon opening it, I see several 1's.

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There is no easy way to do what you are trying to do. The PLC has no way of knowing what it is controlling. The best way, would be to add logic that will track the sequence until sequence complete, then reset. If the sequence does not complete, you can look at the added logic to see where in the sequence the machine stopped. Trying to find out what rung last evaluated as true probably would not lead you to any kind of useful information. As was mentioned earlier, The PLC is scanning the ladder at a fast rate, continuously, regardless of if the machine is running or not. Edited by 93lt1

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Something I have done in the past for troubleshooting sequence logic, is create a custom data monitor in RSLogix. I put in the addresses for bits that start a sequence, the sequence done bits, as well as any safety status or interlocking bits that could stop the sequence. Then you just go online and look at the CDM. For example, Step 1 was enabled, it went done, step 2 enabled, but did not go done. Now I only need to look at the logic concerning transition from Step 1 to Step 2.

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Thanks everyone... I figured it out. Seems like whoever programmed these tools must have thought like mgvol, because they (the programs) are mostly setup like his suggestion.

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